Active material and positive electrode and lithium-ion second battery using same

ABSTRACT

A method for manufacturing an active material containing a triclinic LiVOPO 4  crystal particle that has a spherical form and an average particle size of 20 to 200 nm. The method includes a step of manufacturing the crystal particle by hydrothermal synthesis.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This is a continuation of application Ser. No. 12/568,075 filed Sep. 28,2009, and claims the benefit of Japanese Application No. 2008-254372filed Sep. 30, 2008. The entire disclosures of the prior applicationsare hereby incorporated by reference herein in their entirety.

BACKGROUND

The present invention relates to an active material, and a positiveelectrode and lithium-ion secondary battery using the same.

Laminar oxides (LiCoO₂, LiNiO₂, LiNiMnCoO₂, etc.), spinel compounds(LiMn₂O₄, etc.), and lithium-containing phosphates (LiFePO₄, etc.) haveconventionally been known as positive electrode active materials forlithium-ion secondary batteries.

On the other hand, Li has been known to be reversibly inserted into andde-inserted from crystals represented by structural formulas of VOPO₄and LiVOPO₄ (see Japanese Patent Application Laid-Open No. 2003-68304;N. Dupre et al., Solid State Ionics, 140, pp. 209-221 (2001); and N.Dupre et al., J. Power Sources, 97-98, pp. 532-534 (2001)). LiVOPO₄ hasbeen under study as a material substituting for the above-mentionedlaminar oxide or spinel compound, since it can enhance the safety ofbatteries and the stability of battery characteristics. While LiVOPO₄takes a number of crystal systems, electrochemical characteristics ofits orthorhombic crystal (β type) and triclinic crystal (a type) havebeen reported. Japanese Patent Application Laid-Open No. 2004-303527discloses a method of improving a rate characteristic of LiVOPO₄ byincreasing its specific surface area with orthorhombic LiVOPO₄ crystalshaving a median size of 20 μm or less.

SUMMARY OF THE INVENTION

However, orthorhombic LiVOPO₄ crystals such as those described inJapanese Patent Application Laid-Open No. 2004-303527 are problematic inthat they are likely to cause a decrease in capacity when charging anddischarging are repeated, thus being inferior in cycle characteristic.On the other hand, triclinic LiVOPO₄ crystals are problematic in thatthey are inferior to the orthorhombic LiVOPO₄ crystals in terms ofcrystal symmetry, thus being harder to obtain favorable Li-ionconductivity, thereby exhibiting a lower rate characteristic.

In view of the above-mentioned problems of the prior art, it is anobject of the present invention to provide an active material whichcontains a crystal of LiVOPO₄ and can attain a favorable cyclecharacteristic and a favorable rate characteristic at the same time, anda positive electrode and lithium-ion secondary battery using the same.

For achieving the above-mentioned object, the present invention providesan active material containing a triclinic LiVOPO₄ crystal particle,while the crystal particle has a spherical form and an average particlesize of 20 to 200 nm.

Thus constructed active material can attain a favorable cyclecharacteristic and a favorable rate characteristic at the same time andyield a sufficient capacity. This active material is presumed to haveimproved the rate characteristic, which is a drawback of the triclinicLiVOPO₄ crystals, by speeding up Li-ion diffusion with a fine sphericalparticle having the above-mentioned average particle size employed as atriclinic LiVOPO₄ crystal particle. It is also inferred that a smallersurface area reduces unnecessary reactions with electrolytic solutions,thereby improving the cycle characteristic.

Preferably, in the active material of the present invention, the crystalparticle has a BET specific surface area of 2 to 50 m²/g. This seems toreduce unnecessary reactions with electrolytic solutions, therebyimproving the cycle characteristic.

Preferably, in the active material of the present invention, the crystalparticle has a longer to shorter diameter ratio (longer diameter/shorterdiameter) of 1 to 2. This makes it possible to adjust the BET specificsurface area to an optimal value.

The present invention also provides a positive electrode containing theactive material of the present invention. By containing the activematerial of the present invention, this positive electrode can attain afavorable cycle characteristic and a favorable rate characteristic atthe same time and yield a sufficient capacity.

The present invention further provides a lithium-ion secondary batteryincluding the positive electrode of the present invention. By includingthe positive electrode containing the active material of the presentinvention, this lithium-ion secondary battery can attain a favorablecycle characteristic and a favorable rate characteristic at the sametime and yield a sufficient capacity.

As in the foregoing, the present invention can provide an activematerial which contains a crystal of LiVOPO₄ and can attain a favorablecycle characteristic and a favorable rate characteristic at the sametime, and a positive electrode and lithium-ion secondary battery usingthe same.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] is a front view illustrating a preferred embodiment of thelithium-ion secondary battery in accordance with the present invention;

[FIG. 2] is an unfolded view of the inside of the lithium-ion secondarybattery illustrated in FIG. 1 as seen in a direction normal to a surfaceof a negative electrode 10;

[FIG. 3] is a schematic sectional view of the lithium-ion secondarybattery taken along the line X1-X1 of FIG. 1;

[FIG. 4] is a schematic sectional view illustrating a main part of thelithium-ion secondary battery taken along the line X2-X2 of FIG. 1;

[FIG. 5] is a schematic sectional view illustrating a main part of thelithium-ion secondary battery taken along the line Y-Y of FIG. 1;

[FIG. 6] is a schematic sectional view illustrating an example of basicstructures of the negative electrode in the lithium-ion secondarybattery illustrated in FIG. 1;

[FIG. 7] is a schematic sectional view illustrating an example of basicstructures of the positive electrode in the lithium-ion secondarybattery illustrated in FIG. 1;

[FIG. 8] is a partly broken perspective view illustrating anotherpreferred embodiment of the lithium-ion secondary battery in accordancewith the present invention;

[FIG. 9] is a schematic sectional view of the lithium-ion secondarybattery taken along the YZ plane of FIG. 8; and

[FIG. 10] is an electron micrograph of spherical triclinic LiVOPO₄crystal particles obtained by Example 1 (magnification: ×100,000).

DESCRIPTION OF EMBODIMENTS

In the following, preferred embodiments of the present invention will beexplained in detail with reference to the drawings as the case may be.In the drawings, the same or equivalent parts will be referred to withthe same signs while omitting their overlapping descriptions. Ratios ofdimensions in the drawings are not limited to those depicted.

Active Material and Method of Manufacturing the Same

The active material of the present invention contains a triclinicLiVOPO₄ crystal particle, while the crystal particle has a sphericalform and an average particle size of 20 to 200 nm.

In the crystal particle, the average particle size is required to be 20to 200 nm and is preferably 25 to 150 nm, more preferably 30 to 100 nm.When the average particle size is less than 20 nm, the discharge voltagedecreases, thereby lowering the capacity. When the average particle sizeexceeds 200 nm, on the other hand, the rate characteristic deteriorates.

The longer to shorter diameter ratio (longer diameter/shorter diameter)of the crystal particle is preferably 1 to 2, more preferably 1.0 to1.8. The longer to shorter diameter ratio of the crystal particle closerto 1 is more preferred, since the crystal particle attains a form closerto a true sphere. When the longer to shorter diameter ratio exceeds 2,the cycle characteristic tends to deteriorate.

Here, the shorter diameter of the crystal particle refers to thedistance between two parallel lines tangential to the outer periphery ofthe crystal particle which yield the shortest distance therebetweenwhile sandwiching the particle. On the other hand, the longer diameterof the crystal particle refers to the distance between two parallellines tangential to the outer periphery of the crystal particle whichyield the longest distance therebetween while sandwiching the particleand being perpendicular to the former parallel lines defining theshorter diameter. The longer to shorter diameter ratio (longerdiameter/shorter diameter) in the present invention is determined as anaverage of respective measured values of longer to shorter diameterratio (longer diameter/shorter diameter) in given 10 crystal particlesin an electron micrograph of crystal particles. The average particlesize in the present invention is determined as an average of respectivemeasured values of longer diameter in given 10 crystal particles in anelectron micrograph of crystal particles.

The BET specific surface area of the crystal particle is preferably 2 to50 m²/g, more preferably 2.2 to 35 m²/g, further preferably 2.5 to 20m²/g. The cycle characteristic tends to deteriorate when the BETspecific surface area of the crystal particle is less than 2 m²/g ormore than 50 m²/g. The BET specific surface area is determined from anitrogen adsorption isotherm by using the BET adsorption isothermequation.

The active material of the present invention may consist of theabove-mentioned crystal particle alone or be a composite material of thecrystal particle and other components. Examples of the compositematerial include one in which a surface of the crystal particle iscoated with electrically conductive layer which consists of carbon orthe like and one in which a conductive auxiliary such as carbon black iscarried by the crystal particle. As the other components forming thecomposite material, materials used in positive electrodes of lithium-ionsecondary batteries can be employed without any restrictions inparticular.

Preferably, in the active material of the present invention, thespherical triclinic LiVOPO₄ crystal particle is manufactured byhydrothermal synthesis. Manufacturing the crystal particle byhydrothermal synthesis makes it possible to control the particle size,so as to yield the spherical triclinic LiVOPO₄ crystal particle havingthe above-mentioned average particle size. Thus manufactured sphericaltriclinic LiVOPO₄ crystal particle has a higher capacity and superiorrate and cycle characteristics as compared with conventional triclinicLiVOPO₄ crystals.

Conventionally known methods of synthesizing triclinic LiVOPO₄ crystalsinclude a method of mixing, pulverizing, and firing a solid to become amaterial and a method of dissolving a material into water andevaporating the mixture to dryness. However, it is difficult for theseconventional synthesizing methods to manufacture fine sphericaltriclinic LiVOPO₄ crystal particles having the above-mentioned averageparticle size.

The spherical triclinic LiVOPO₄ crystal particle in the presentinvention can be manufactured by the following procedure, for example,by hydrothermal synthesis.

That is, the triclinic LiVOPO₄ crystal particle can be obtained byheat-processing at least an aqueous phosphate compound solution and avanadium-containing compound which are sealed in a closed container suchas an autoclave (first heat treatment step), adding at least anLi-containing compound to the resulting product, further heat-processingthe mixture (second heat treatment step), and pulverizing the resultingproduct. Since thus obtained crystal particle may include crystalparticles which are not spherical and those having a relatively largeparticle size depending on manufacturing conditions, the powder obtainedby the foregoing steps is heated (fired) at a temperature of at least450° C. but not higher than 600° C. in an air atmosphere, so as to growa spherical structure preferentially (firing step), and then isclassified by airflow classification or the like (classification step),whereby the spherical triclinic LiVOPO₄ crystal particle having thetarget particle size can be obtained.

Here, an aqueous H₃PO₄ solution is preferred as the aqueous phosphatecompound solution. V₂O₅ is preferred as the vanadium-containingcompound. LiOH.H₂O is preferred as the Li-containing compound.

In the first heat treatment step, the heating temperature is preferably60 to 150° C., more preferably 80 to 120° C. Such a heating temperaturetends to allow a uniform precursor to be made. The heat treatment ispreferably carried out for 1 to 30 hr, more preferably 3 to 20 hr. Sucha heat treatment time tends to allow a uniform precursor to be made.Preferably, the first heat treatment step is carried out while stirringthe material.

In the second heat treatment step, the heating temperature is preferably150 to 200° C., more preferably 160 to 185° C. Such a heatingtemperature tends to allow a spherical triclinic LiVOPO₄ crystal havinga large charge/discharge capacity to be made. The heat treatment ispreferably carried out for 3 to 20 hr, more preferably 5 to 15 hr. Sucha heating time tends to allow a spherical triclinic LiVOPO₄ crystalhaving a large charge/discharge capacity to be made. The second heattreatment step is also preferably carried out while stirring thematerial. A step of washing impurities away from the powder may beprovided after the second heat treatment step. Under some conditions,viscous matters may be formed in the mixture in the first and secondheat treatment steps. Such viscous matters can be washed with a solventsuch as water and dried by heating at a temperature at least lower thanthe second heat treatment temperature in the washing step.

In the firing step, the heating temperature is preferably 450 to 600°C., more preferably 480 to 580° C. Such a heating temperature tends tomake it easier to yield the spherical triclinic LiVOPO₄ crystal. Thefiring is preferably carried out for 1 to 20 hr, more preferably 2 to 10hr. Such a firing time tends to be able to sufficiently grow crystalshaving a spherical structure.

The classifying step can be carried out by centrifugal airflowclassification. The classifying step makes it possible to yield acrystal particle having a desirable form and particle size. Theclassifying step allows crystal particles having a spherical form whoseaverage particle size falls within the range of 20 to 200 nm to beobtained efficiently and reliably.

When forming a composite material of the triclinic LiVOPO₄ crystal andother components as the active material of the present invention, itwill be preferred if the other components to form the composite areadded in at least one of the first and second heat treatment steps. Forexample, adding a carbon particle or carbon source in the first orsecond heat treatment step can yield a composite material of thetriclinic LiVOPO₄ crystal and carbon particle or a composite material inwhich a carbon layer is formed on the surface of the triclinic LiVOPO₄crystal. Carbon black is preferred as the carbon particle. As the carbonsource, organic acids and alcohols are preferred; ascorbic acid, whichis an organic acid, is more preferred.

Adding the carbon source in at least one of the first and second heattreatment steps is also preferred, since the particle growth issuppressed by carbon covering the surface of the crystal particle,whereby finer crystal particles can be obtained.

Lithium-Ion Secondary Battery and Method of Manufacturing the Same

FIG. 1 is a front view illustrating a preferred embodiment of thelithium-ion secondary battery in accordance with the present invention.FIG. 2 is an unfolded view of the inside of the lithium-ion secondarybattery illustrated in FIG. 1 as seen in a direction normal to a surfaceof a negative electrode 10. FIG. 3 is a schematic sectional view of thelithium-ion secondary battery taken along the line X1-X1 of FIG. 1. FIG.4 is a schematic sectional view illustrating a main part of thelithium-ion secondary battery taken along the line X2-X’of FIG. 1. FIG.5 is a schematic sectional view illustrating a main part of thelithium-ion secondary battery taken along the line Y-Y of FIG. 1.

As illustrated in FIGS. 1 to 5, the lithium-ion secondary battery 1 ismainly constituted by a planar negative electrode 10 and a planarpositive electrode 20 which oppose each other, a planar separator 40arranged between and adjacent to the negative and positive electrodes10, 20, an electrolytic solution containing a lithium ion, a case (outerpackage) 50 accommodating them in a closed state, a negative electrodelead 12 having one end part electrically connected to the negativeelectrode 10 and the other end part projecting out of the case 50, and apositive electrode lead 22 having one end part electrically connected tothe positive electrode 20 and the other end part projecting out of thecase 50. The positive electrode 20 contains the above-mentioned activematerial of the present invention.

In this specification, the “negative electrode”, which is based on thepolarity of the battery at the time of discharging, refers to anelectrode which releases electrons by an oxidation reaction at the timeof discharging. The “positive electrode”, which is based on the polarityof the battery at the time of discharging, refers to an electrode whichreceives electrons by a reduction reaction at the time of discharging.

Constituents of this embodiment will now be explained in detail withreference to FIGS. 1 to 7.

First, the negative and positive electrodes 10, 20 will be explained.FIG. 6 is a schematic sectional view illustrating an example of basicstructures of the negative electrode 10 in the lithium-ion secondarybattery 1 illustrated in FIG. 1. FIG. 7 is a schematic sectional viewillustrating an example of basic structures of the positive electrode 20in the lithium-ion secondary battery 1 illustrated in FIG. 1.

As illustrated in FIG. 6, the negative electrode 10 is constituted by acurrent collector 16 and a negative electrode active material containinglayer 18 formed on the current collector 16. As illustrated in FIG. 7,the positive electrode 20 is constituted by a current collector 26 and apositive electrode active material containing layer 28 formed on thecurrent collector 26.

The current collectors 16, 26 are not limited in particular as long asthey are good conductors which can sufficiently transfer electriccharges to the negative and positive electrode active materialcontaining layers 18, 28, respectively; known current collectorsemployed in lithium-ion secondary batteries can be used. Examples of thecurrent collectors 16, 26 include metal foils made of copper andaluminum, respectively.

The negative electrode active material containing layer 18 of thenegative electrode 10 is mainly constituted by a negative electrodeactive material and a binder. Preferably, the negative electrode activematerial containing layer 18 further contains a conductive auxiliary.

The negative electrode active material is not limited in particular aslong as it allows occlusion and release of lithium ions, desorption andinsertion (intercalation) of lithium ions, or doping and undoping oflithium ions and their counteranions (e.g., PF₆ ⁻ and ClO₄ ⁻) to proceedreversibly; known negative electrode active materials can be used.Examples of the negative electrode active material include carbonmaterials such as natural graphite, synthetic graphite, non-graphitizingcarbon, graphitizable carbon, and low-temperature-finable carbon; metalssuch as Al, Si, and Sn which are combinable with lithium; amorphouscompounds mainly composed of oxides such as SiO, SiO₂, SiO_(>), andSnO₂; lithium titanate (Li₄Ti₅O₁₂); and TiO₂.

As the binder used in the negative electrode 10, known binders can beemployed without any restrictions in particular. Examples includefluororesins such as polyvinylidene fluoride (PVDF),polytetrafluoroethylene (PTFE), tetrafluoroethylene/hexafluoropropylenecopolymers (FEP), tetrafluoroethylene/perfluoroalkylvinyl ethercopolymers (PFA), ethylene/tetrafluoroethylene copolymers (ETFE),polychlorotrifluoroethylene (PCTFE), ethylene/chlorotrifluoroethylenecopolymers (ECTFE), and polyvinyl fluoride (PVF). The binder not onlybinds constituent materials such as active material particles, theconductive auxiliary added when necessary, and the like together, butalso contributes to binding these constituent materials to the currentcollector.

Other examples of the binder include fluorine rubbers based onvinylidene fluoride such as fluorine rubbers based on vinylidenefluoride/hexafluoropropylene (VDF/HFP-based fluorine rubbers).

Still other examples of the binder include polyethylene, polypropylene,polyethylene terephthalate, aromatic polyamides, cellulose,styrene/butadiene rubber, isoprene rubber, butadiene rubber, andethylene/propylene rubber. Also employable are thermoplastic elastomericpolymers such as styrene/butadiene/styrene block copolymers andhydrogenated derivatives thereof, styrene/ethylene/butadiene/styrenecopolymers, and styrene/isoprene/styrene block copolymers andhydrogenated derivatives thereof. Further employable are syndiotactic1,2-polybutadiene, ethylene/vinyl acetate copolymers, propylene/α-olefin(having a carbon number of 2 to 12) copolymers, and the like. Conductivepolymers may also be used.

As the conductive auxiliary used when necessary, known conductiveauxiliaries can be used without any restrictions in particular. Examplesinclude carbon blacks, carbon materials, fine powders of metals such ascopper, nickel, stainless steel, and iron, mixtures of the carbonmaterials and fine metal powders, and conductive oxides such as ITO.

The content of the negative electrode active material in the negativeelectrode active material containing layer 18 is preferably 80 to 97mass %, more preferably 85 to 96 mass %, based on the total amount ofthe negative electrode active material containing layer 18. When theactive material content is less than 80 mass %, the energy density tendsto become lower than that in the case where the content falls within therange mentioned above. When the active material content exceeds 97 mass%, the bonding force tends to become weaker, thereby lowering the cyclecharacteristic as compared with the case where the content falls withinthe range mentioned above.

The positive electrode active material containing layer 28 is mainlyconstituted by a positive electrode active material and a binder.Preferably, the positive electrode active material containing layer 28further contains a conductive auxiliary.

While the positive electrode active material is not limited inparticular as long as it allows occlusion and release of lithium ions,desorption and insertion (intercalation) of lithium ions, or doping andundoping of lithium ions and their counteranions (e.g., ClO₄ ⁻) toproceed reversibly, the lithium-ion secondary battery of the presentinvention contains at least the above-mentioned active material of thepresent invention as the positive electrode active material.

When necessary, known positive electrode active materials may be used inaddition to the active material of the present invention as the positiveelectrode active material. Examples of such positive electrode activematerials include lithium cobaltate (LiCoO₂), lithium nickelate(LiNiO₂), lithium manganese spinel (LiMn₂O₄), mixed metal oxidesexpressed by the general formula of LiNi_(x)Co_(y)Mn_(z)M₆O₂ (wherex+y+z+a=1, 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦a≦1, and M is at least one kind ofelement selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr), a lithiumvanadium compound (LiV₂O₅), olivine-type LiMPO₄ (where M is at least onekind of element selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr, orVO), and mixed metal oxides such as lithium titanate (Li₄Ti₅O₁₂).

As the binder used in the positive electrode 20, one similar to thebinder used in the negative electrode 10 can be employed. As theconductive auxiliary used in the positive electrode 20 when necessary,one similar to the conductive auxiliary used in the negative electrode10 can be employed.

The current collector 26 of the positive electrode 20 is electricallyconnected to one end of the positive electrode lead 22 made of aluminum,for example, while the other end of the positive electrode lead 22extends to the outside of the case 50. On the other hand, the currentcollector 16 of the negative electrode 10 is electrically connected toone end of the negative electrode lead 12 made of copper or nickel, forexample, while the other end of the negative electrode lead 12 extendsto the outside of the case 50.

The separator 40 arranged between the negative and positive electrodes10, 20 is not limited in particular as long as it is formed from aporous body which is ionically permeable but electrically insulative;known separators used in lithium-ion secondary batteries can beemployed. Examples include multilayer bodies of films made of any ofpolyethylene, polypropylene, and polyolefin; drawn films of mixtures ofthe polymers mentioned above; and fibrous nonwovens made of at least oneconstituent material selected from the group consisting of cellulose,polyester, and polypropylene.

The electrolytic solution (not depicted) fills the inner space of thecase 50 and is partly contained within the negative electrode 10,positive electrode 20, and separator 40. Employed as the electrolyticsolution is a nonaqueous electrolytic solution in which a lithium saltis dissolved in an organic solvent. As the lithium salt, salts such asLiPF₆, LiClO₄, LiBF₄, LiAsF₆, LiCF₃SO₃, LiCF₃CF₂SO₃, LiC(CF₃SO₂)₃,LiN(CF₃SO₂)₂, LiN(CF₃CF₂SO₂)₂, LiN(CF₃SO₂)(C₄F₉SO₂), LiN(CF₃CF₂CO)₂, andthe like can be used, for example. These salts may be used singly or incombinations of two or more. The electrolytic solution may be gelled byaddition of polymers and the like thereto.

As the organic solvent, solvents used in known electrochemical devicescan be employed. Preferred examples include propylene carbonate,ethylene carbonate, and diethyl carbonate. They may be used singly or inmixtures of two or more at any ratios.

The case 50 is formed from a pair of opposing films (first and secondfilms 51, 52). Here, as illustrated in FIG. 2, the first and secondfilms 51, 52 in this embodiment are connected to each other. That is,the case 50 in this embodiment is formed by bending a rectangular filmmade of a single composite package film at a bending line X3-X3illustrated in FIG. 2, overlaying a pair of opposing edge parts of therectangular film (an edge part 51B of the first film 51 and an edge part52B of the second film 52 in the drawing) on each other, and bondingthem together by an adhesive or heat-sealing. Here, 51A in FIGS. 1 and 2and 52A in FIG. 2 refer to partial regions which are not bonded or notheat-sealed in the first and second films 51, 52.

The first and second films 51, 52 indicate film parts having respectivesurfaces opposing each other when a single rectangular film is bent asmentioned above. In this specification, respective edge parts of thefirst and second films 51, 52 after they are bonded together arereferred to as “seal parts”.

The film constituting the first and second films 51, 52 is a flexiblefilm as mentioned above. Though this film is not limited in particularas long as it is a flexible film, it is preferably a “composite packagefilm” having at least an innermost layer made of a polymer in contactwith a power generating element 60 and a metal layer arranged on theside of the innermost layer opposite from the side in contact with thepower generating element.

As illustrated in FIGS. 1 and 2, the part of the negative electrode lead12 in contact with the seal parts of the outer bag constructed by theedge parts 51B, 52B of the first and second films 51, 52 is covered withan insulator 14 for preventing the negative electrode lead 12 fromcoming into contact with the metal layer in the composite package filmconstituting the films. The part of the positive electrode lead 22 incontact with the seal parts of the outer bag constructed by the edgeparts 51B, 52B of the first and second films 51, 52 is covered with aninsulator 24 for preventing the positive electrode lead 22 from cominginto contact with the metal layer in the composite package filmconstituting the films.

The insulators 14, 24 may be formed from polymers, for example, thoughtheir structures are not restricted in particular. The insulators 14, 24may be omitted when the negative and positive electrode leads 12, 22 canfully be prevented from coming into contact with the metal layers intheir corresponding metal layers in the composite package film.

A method of manufacturing the above-mentioned lithium-ion secondarybattery 1 will now be explained.

For making the power generating element 60 (multilayer body in which thenegative electrode 10, separator 40, and positive electrode 20 arelaminated in this order), known methods employed for manufacturinglithium-ion secondary batteries can be used without any restrictions inparticular.

First, when making the negative and positive electrodes 10, 20, theconstituents mentioned above are mixed and dispersed into a solvent inwhich the binder is soluble, so as to produce an electrode formingcoating liquid (slurry, paste, or the like). The solvent is not limitedin particular as long as the binder is soluble therein; examples includeN-methyl-2-pyrrolidone and N,N-dimethylformamide.

Subsequently, the electrode forming coating liquid is applied onto acurrent collector surface, dried, and extended, so as to form an activematerial containing layer on the current collector, thereby completingthe making of the negative and positive electrodes 10, 20. The techniquefor applying the electrode forming coating liquid to the currentcollector surface is not limited in particular, but may be determined asappropriate according to the material, form, and the like of the currentcollector. Examples of the coating method include metal mask printing,electrostatic coating, dip coating, spray coating, roll coating, doctorblading, gravure coating, and screen printing.

Thereafter, the negative and positive leads 12, 22 are electricallyconnected to thus prepared negative and positive electrodes 10, 20,respectively.

Subsequently, the separator 40 is arranged between the negative andpositive electrodes 10, 20 so as to be in contact therewith (preferablyin an unbonded state), thus completing the power generating element 60.Here, the surface F2 of the negative electrode 10 on the negativeelectrode active material containing layer 18 side and the surface F2 ofthe positive electrode 20 on the positive electrode active materialcontaining layer 28 side are arranged so as to be in contact with theseparator 40.

An example of methods of making the case 50 will now be explained.First, for constructing the first and second films from theabove-mentioned composite package film, a known manufacturing methodsuch as dry lamination, wet lamination, hot melt lamination, orextrusion lamination is used.

For example, a film to become a polymer layer and a metal foil made ofaluminum or the like, which constitute the composite package film, areprepared. The metal foil can be prepared by extending a metal material,for example.

Subsequently, the metal foil is attached onto the film to become thepolymer layer with an adhesive interposed therebetween, and so forth, soas to make a composite package film (multilayer film) preferably havingthe above-mentioned structure composed of a plurality of layers. Then,the composite package film is cut into a predetermined size, so as toprepare a rectangular film.

Next, as previously explained with reference to FIG. 2, the single filmis bent, and the seal part 51B (edge part 51B) of the first film 51 andthe seal part 52B (edge part 52B) of the second film 52 are heat-sealedwith a sealer, for example, by a desirable seal width under apredetermined heating condition. Here, for securing an opening forintroducing the power generating element 60 into the case 50, a partwithout heat-sealing is provided. This yields the case 50 with theopening.

Then, the power generating element 60 having the negative and positiveelectrode leads 12, 22 electrically connected thereto is inserted intothe case 50 having the opening. Subsequently, the electrolytic solutionis injected therein. Thereafter, while the negative and positiveelectrodes 12, 22 are partly inserted in the case 50, the opening of thecase 50 is sealed with the sealer. This completes the making of the case50 and lithium-ion secondary battery 1. The lithium-ion secondarybattery of the present invention is not limited to the form illustratedin FIG. 1, but may have a cylindrical faun, for example.

Though a preferred embodiment of the present invention is explained inthe foregoing, the present invention is not limited thereto. Forexample, in the explanation of the above-mentioned embodiment, the sealpart of the lithium-ion secondary battery 1 may be bent, so as to yielda more compact structure. Though the above-mentioned embodiment explainsthe lithium-ion secondary battery 1 comprising one each of the negativeand positive electrodes 10, 20, two or more each of the negative andpositive electrodes 10, 20 may be provided while always arranging oneseparator 40 between each pair of the negative and positive electrodes10, 20.

Another preferred embodiment of the lithium-ion secondary battery inaccordance with the present invention will now be explained.

FIG. 8 is a partly broken perspective view illustrating the lithium-ionsecondary battery 100 in accordance with this embodiment. FIG. 9 is asectional view taken along the YZ plane of FIG. 8. As illustrated inFIGS. 8 and 9, the lithium-ion secondary battery 100 in accordance withthis embodiment is mainly constituted by a multilayer structure 85, acase (outer package) 50 for accommodating the multilayer structure 85 ina closed state, and negative and positive leads 12, 22 for connectingthe multilayer structure 85 to the outside of the case 50.

As illustrated in FIG. 9, the multilayer structure 85 is one in which athree-layer negative electrode 130, a separator 40, a three-layerpositive electrode 140, a separator 40, a three-layer negative electrode130, a separator 40, a three-layer positive electrode 140, a separator40, and a three-layer negative electrode 130 are laminated in this orderfrom the upper side.

Each three-layer negative electrode 130 has a current collector(negative electrode current collector) 16 and two negative electrodeactive material containing layers 18 formed on respective surfaces ofthe current collector 16. The three-layer negative electrode 130 islaminated such that the negative electrode active material containinglayer 18 is in contact with the separator 40.

Each three-layer positive electrode 140 has a current collector(positive electrode current collector) 26 and two positive electrodeactive material containing layers 28 formed on respective surfaces ofthe current collector 26. The three-layer positive electrode 140 islaminated such that the positive electrode active material containinglayer 28 is in contact with the separator 40.

The electrolytic solution (not depicted) fills the inner space of thecase 50 and is partly contained within the negative electrode activematerial containing layers 18, positive electrode active materialcontaining layers 28, and separators 40.

As illustrated in FIG. 8, respective ends of the current collectors 16,26 are formed with tongues 16 a, 26 a extending outward. As illustratedin FIG. 8, the negative and positive electrode leads 12, 22 project fromwithin the case 50 to the outside through a seal part 50 b. An end partof the lead 12 within the case 50 is welded to the respective tongues 16a of the three current collectors 16, so that the lead 12 iselectrically connected to the negative electrode active materialcontaining layers 18 through the current collectors 16. On the otherhand, an end part of the lead 22 within the case 50 is welded to therespective tongues 26 a of the two current collectors 26, so that thelead 22 is electrically connected to the positive electrode activematerial containing layers 28 through the current collectors 26.

In the leads 12, 22, the parts held by the seal part 50 b of the case 50are coated with insulators 14, 24 made of a resin or the like in orderto enhance sealability as illustrated in FIG. 8. The leads 12, 22 areseparated from each other in a direction orthogonal to the laminatingdirection of the multilayer structure 85.

As illustrated in FIG. 8, the case 50, which is formed by folding arectangular flexible sheet 51C into two at substantially thelongitudinal center part thereof, holds the multilayer structure 85 fromboth sides in the laminating direction (vertical direction). Among endparts of the sheet 51C folded into two, the seal parts 50 b in the threesides excluding the folded part 50 a are bonded by heat-sealing or anadhesive, whereby the multilayer structure 85 is hermetically sealedwithin the case 50. The case 50 is also bonded to the insulators 14, 24at the seal part 50 b, so as to seal the leads 12, 22.

As the current collectors 16, 26, active material containing layers 18,28, separators 40, electrolytic solution, leads 12, 22, insulators 14,24, and case 50 in the lithium-ion secondary battery 100 illustrated inFIGS. 8 and 9, those made of constituent materials similar to theirequivalents in the lithium-ion secondary battery 1 illustrated in FIGS.1 to 7 are used.

When the multilayer structure 85 has a multilayer structure ofthree-layer negative electrode/separator/three-layer positiveelectrode/separator/three-layer negative electrode, i.e., both outermostlayers are negative electrodes, it tends to suppress heating moreeffectively at the time of a nail penetration test. This effect isobtained as long as the multilayer structure 85 has a multilayerstructure of negative electrode/separator/(positiveelectrode/separator/negative electrode)_(n), where n is an integer of 1or greater.

Though the lithium-ion secondary battery 100 illustrated in FIGS. 8 and9 has four sets of secondary batteries, i.e., four sets of negativeelectrode/separator/positive electrode, as single cells, the number ofsets may be more or less than 4.

Though the above-mentioned embodiment illustrates a mode in which eachof the two outermost layers is a three-layer negative electrode 130 as apreferred mode, one or both of the outermost layers may be a two-layernegative electrode.

Though the above-mentioned embodiment illustrates a mode in which eachof the two outermost layers is a negative electrode as a preferred mode,the two outermost layers may be positive and negative electrodes or bothpositive electrodes.

EXAMPLES

The present invention will now be explained more specifically withreference to an example and a comparative example, but is not limited tothe following example.

Example 1 Making of Active Material

An aqueous H₃PO₄ solution in which 23.08 g of H₃PO₄ were dissolved in500 g of water was introduced into a 1.5-L autoclave container, and V₂O₅(18.37 g) was gradually added thereto. After completely adding V₂O₅, thecontainer was hermetically closed, and reflux was carried out for 16 hrat 95° C. with stirring at 200 rpm. After completing the reflux, thecontainer was once opened when its temperature dropped to roomtemperature, and LiOH.H₂O (8.48 g) and C₆H₈O₆ (7.13 g) were graduallyadded. Thereafter, the container was hermetically closed again and heldat 160° C. for 8 hr. Thus obtained mixture was washed with about 300 mlof water added thereto. The washed product was heated at 90° C. forabout 23 hr in an oven and then pulverized by high-speed rotarypulverization, thus yielding a gray powder.

Thus obtained powder was put into an alumina crucible and heated fromroom temperature to 550° C. in 45 min. After being heat-treated for 4 hrat 550° C., the powder was rapidly cooled, whereby a brownish-red firedpowder was obtained. The resulting fired powder was classified into therange of 0.05 to 0.1 μm by centrifugal airflow classification, wherebyspherical crystal particles were obtained as an active material. X-raypowder diffraction patterns of the crystal particles were measured and,as a result, it was verified that the obtained crystal particles weretriclinic (α type) LiVOPO₄ crystals exhibiting the X-ray diffractionpattern described in JCPDS card 72-2253.

FIG. 10 is an electron micrograph of thus obtained triclinic LiVOPO₄crystal particles (magnification: ×100,000). As illustrated in FIG. 10,the obtained triclinic LiVOPO₄ crystal particles had spherical forms.The particle size (longer diameter) and the longer to shorter diameterratio (longer diameter/shorter diameter) of each of given 10 crystalparticles were measured by electron microscopic observation, and averagevalues were determined. As a result, the average particle size was 92mm, the longer to shorter diameter ratio (average value) was 1.6, andthe BET specific surface area was 7.5 m²/g.

Making of Lithium-Ion Secondary Battery

A mixture of 84 parts by mass of the above-mentioned active material, 8parts by mass of acetylene black, and 8 parts by mass of polyvinylidenefluoride (PVdF) was dissolved into N-methylpyrrolidone (NMP), so as toyield a slurry-like positive electrode coating liquid. This coatingliquid was applied to an Al foil by doctor blading and then dried, so asto form a positive electrode active material containing layer. Thisyielded a positive electrode in which a current collector having athickness of 15 μm and an active material containing layer having athickness of 50 μm were laminated.

A mixture of 92 parts by mass of natural graphite (product name: OMAC,manufactured by Osaka Gas Chemicals Co., Ltd.) and 8 parts by mass ofpolyvinylidene fluoride (PVdF) was dissolved into NMP, so as to yield aslurry-like negative electrode coating liquid. This coating liquid wasapplied to an Al foil by doctor blading and then dried, so as to form anegative electrode active material containing layer. This yielded anegative electrode in which a current collector having a thickness of 15μm and an active material containing layer having a thickness of 45 μmwere laminated.

A mixed solvent was obtained by mixing 20 parts by volume of propylenecarbonate (PC), 10 parts by volume of ethylene carbonate (EC), and 70pars by volume of diethyl carbonate. Lithium hexafluorophosphate (LiPF₆)was dissolved into the mixed solvent such as to yield a concentration of1.5 mol·dm⁻³, whereby an electrolytic solution was obtained.

The above-mentioned negative electrode was punched into a size of 17.5mm×34.5 mm, while the above-mentioned positive electrode was punchedinto a size of 17 mm×34 mm, and a separator made of polyethylene wasarranged between and laminated with the negative and positiveelectrodes, so as to form a battery matrix. Thus obtained battery matrixwas put into an aluminum-laminated film, the above-mentionedelectrolytic solution was injected therein, and the film was sealedunder vacuum. A lithium-ion secondary battery was made by the foregoingprocedure.

Comparative Example 1 Making of Active Material

LiNO₃, V₂O₅, and H₃PO₄ were dissolved into water at a molar ratio of2:1:2. Thus obtained solution was evaporated to dryness at 25° C., driedfor 20 hr at 25° C., and then pulverized in a mortar.

The resulting pulverized product was put into an alumina crucible andheated from room temperature to 700° C. in 45 min. After beingheat-treated for 4 hr at 700° C., the product was rapidly cooled,whereby a fired powder was obtained. The resulting fired powder wasclassified into the range of 0.5 to 1 μm by centrifugal airflowclassification, whereby spherical crystal particles were obtained as anactive material. X-ray powder diffraction patterns of the crystalparticles were measured and, as a result, it was verified that theobtained crystal particles were triclinic (a type) LiVOPO₄ crystalsexhibiting the X-ray diffraction pattern described in JCPDS card72-2253. The particle size (longer diameter) of each of given 10spherical crystal particles was measured by electron microscopicobservation, and an average value was determined. As a result, theaverage particle size was 558 nm.

Making of Lithium-Ion Secondary Battery

A mixture of 84 parts by mass of the above-mentioned active material, 8parts by mass of acetylene black, and 8 parts by mass of polyvinylidenefluoride (PVdF) was dissolved into N-methylpyrrolidone (NMP), so as toyield a slurry-like positive electrode coating liquid. This coatingliquid was applied to an Al foil by doctor blading and then dried, so asto form a positive electrode active material containing layer. Thisyielded a positive electrode in which a current collector having athickness of 15 μm and an active material containing layer having athickness of 50 μm were laminated. Except for using this positiveelectrode, a lithium-ion secondary battery was made as in Example 1.

Measurement of Discharge Capacity

The discharge capacity of each of the lithium-ion secondary batteriesobtained by the above-mentioned Example and Comparative Example wasmeasured by performing constant current constant voltage charging to 4.2V with a current equivalent to 0.5 C and then discharging to 2.5 V witha current equivalent to 0.5 C, and a discharge capacity per unit mass ofLiVOPO₄ was calculated. Table 1 lists the results.

Measurement of Rate Characteristic

For each of the lithium-ion secondary batteries obtained by theabove-mentioned Example and Comparative Example, the discharge capacityat 0.1 C (the current value at which constant current discharging at 25°C. completes in 10 hr) and the discharge capacity at 1 C (the currentvalue at which constant current discharging at 25° C. completes in 1 hr)were measured, and the ratio (%) of the discharge capacity at 1 C to thedischarge capacity at 0.1 C was determined as a rate characteristic.Table 1 lists the results.

Measurement of Cycle Characteristic

Each of the lithium-ion secondary batteries obtained by theabove-mentioned Example and Comparative Example was charged at a rate of1 C by CCCV to 4.2 V. Thereafter, constant current discharging wascarried out at a rate of 1 C to 2.5 V. Counting this set of operationsas 1 cycle, 100 cycles were carried out, and the ratio (%) of thedischarge capacity at the 100th cycle to that at the first cycle wasdetermined as a cycle characteristic. Tale 1 lists the result.

TABLE 1 Active material Average Rate particle Discharge characteristicCycle Crystal size capacity (1 C/0.1 C) characteristic system [nm][mAh/g] [%] [%] Example 1 triclinic 92 99 84 89 Comparative triclinic558 82 67 73 Example 1

What is claimed is:
 1. A method for manufacturing an active materialcontaining a triclinic LiVOPO₄ crystal particle, the method comprising:a step of manufacturing the crystal particle by hydrothermal synthesis;wherein the crystal particle has a spherical form and an averageparticle size of 20 to 200 nm.
 2. The method of claim 1, wherein thestep of manufacturing the crystal particle by hydrothermal synthesiscomprises: a first heat treatment step of heat-processing at least anaqueous phosphate compound solution and a vanadium-containing compoundthat are sealed in a closed container; and a second heat treatment stepof adding an Li-containing compound to the resulting product of thefirst heat treatment step and further heat-processing the mixture. 3.The method of claim 1, wherein the crystal particle has a BET specificsurface area of 2 to 50 m²/g.
 4. The method of claim 2, wherein thecrystal particle has a BET specific surface area of 2 to 50 m²/g.
 5. Themethod of claim 1, wherein the crystal particle has a longer to shorterdiameter ratio (longer diameter/shorter diameter) of 1 to
 2. 6. Themethod of claim 2, wherein the crystal particle has a longer to shorterdiameter ratio (longer diameter/shorter diameter) of 1 to
 2. 7. Themethod of claim 3, wherein the crystal particle is a longer to shorterdiameter ratio (longer diameter/shorter diameter) of 1 to
 2. 8. Themethod of claim 4, wherein the crystal particle is a longer to shorterdiameter ratio (longer diameter/shorter diameter) of 1 to
 2. 9. A methodfor manufacturing a positive electrode containing an active material,comprising: a step of manufacturing the active material by the method ofclaim
 1. 10. A method for manufacturing a lithium-ion secondary batteryincluding a positive electrode, comprising: a step of manufacturing thepositive electrode by the method of claim 9.